For a non-aqueous electrolyte solution to be used for lithium ion secondary batteries (hereinafter sometimes referred to simply as “secondary batteries”), a carbonate type solvent such as ethylene carbonate or dimethyl carbonate, has been widely used in that it usually dissolves a lithium salt excellently to provide a high lithium ion conductivity (hereinafter referred to simply as “conductivity”), and it has a wide potential window. However, a carbonate type solvent is flammable and is likely to catch fire by e.g. heat generation of the batteries.
As a method to increase non-flammability (flame retardancy) without deteriorating the performance as a non-aqueous electrolyte, it has been proposed to add a fluorinated solvent (Patent Document 1). However, the fluorinated solvent has a low ability to dissolve the lithium salt and tends to deteriorate the cycle properties.
Under the circumstances, it has been proposed to incorporate a glyme type solvent to a fluorinated solvent having a high non-flammability (flame retardancey) as the main component, to let the lithium salt and the glyme solvent form a complex thereby to obtain a non-aqueous electrolyte solution excellent in the solubility of the lithium salt (Patent Document 2).
Further, a non-aqueous electrolyte solution has been proposed which comprises a fluorinated solvent, a cyclic carbonate, a cyclic carboxylic acid ester and a lithium salt (Patent Documents 3 and 4). However, as a result of a study conducted by the present inventors, it has been found that there has been either problem such that the reactivity with the electrodes is so high that the stability is inadequate, or the conductivity is so low that it is poor for practical application. Thus, it has been difficult to lower the reactivity of the non-aqueous electrolyte solution with the electrodes, while securing the practically sufficient conductivity.
In general, with a secondary battery, the battery temperature will be raised due to Joule heat or external heating, and if the battery temperature reaches such a high temperature that exceeds 150° C., thermal runaway may occur to destroy the battery. As a cause for thermal runaway, it is known that an electrolyte solution is reacted with a positive electrode and a negative electrode, whereby they are decomposed to generate heat. That is, thermal runaway starts as the temperature of a secondary battery has reached a temperature at which the electrolyte solution is reacted with the positive electrode and the negative electrode by e.g. Joule heat, so that they undergo thermal decomposition. Therefore, it is important for a non-aqueous electrolyte solution to be used for a secondary battery that the reactivity with the positive electrode and the negative electrode is low, and heat generation is less likely to occur by a reaction therewith.
In recent years, it has been actively studied to apply secondary batteries to e.g. power sources for vehicles such as electric automobiles which require larger energies, and a non-aqueous electrolyte solution having a lower reactivity with a positive electrode and a negative electrode is desired.
Further, in order to obtain a larger energy efficiently, it is required not only to lower the reactivity of the non-aqueous electrolyte solution with a positive electrode and a negative electrode, but also to have excellent rate properties, initial charge/discharge properties and cycle properties provided.